USB polling (75% done)

Hi,
I've added minimal polling support to the USB P4 repository now. Patch can be
found here:
http://perforce.freebsd.org/chv.cgi?CH=166148
Dumping core to USB disk: Tested and works.
Using USB keyboard in KDB: Does not work because Giant is not locked when
calling into the UKBD's get char routine. UKBD is Giant locked. Someone
familiar with the keyboard system on FreeBSD please step forward and fix this
so that UKBD gets independent of the Giant mutex.
--HPS